Frequency hopping is a technique in which the frequency, typically a carrier frequency, is varied during transmission of data. Frequency hopping has been used in radio communication systems to reduce interference, for example. Frequency hopping may also be used as a modulating method.
Recently communication systems requiring fast frequency hopping capability have been proposed. An example of such a system is an ultra wideband communication system employing OFDMA (Orthogonal Frequency Division Multiple Access) developed by the IEEE 802.15.3 standardization body. In such a system, the required hopping rates may be up to 3 MHz.
Conventional phase locked loop based frequency synthesizers are not able to settle into the desired frequency fast enough. A prior art solution is proposed in A. Batra et al, “Design of a Multiband OFDM System for Realistic UWB Channel Environments”, IEEE Trans. on Microwave Theory and Techniques, vol. 52, No. 9, September 2004. By using a fixed frequency source and a set of frequency dividers it is possible to generate a set of tones, which can then be mixed using a conventional analog mixer, for generating the desired local oscillator (LO) frequency. Fast frequency selection is achieved by switching the selected mixing tones. The prior art solution has several drawbacks. One drawback is the poor spectral purity of the generated LO frequency: harmonics of the low frequency mixing tones are not properly attenuated and they contaminate the generated LO tone with undesired adjacent tones. In addition, the solution requires two single-sideband mixer blocks which leads to high power consumption and a large die area. Furthermore, the prior art generator can generate frequencies of only one band group of the band groups allocated for the IEEE 802.15.3 system.